Photolithography is a fundamental technology in the production of semiconductor devices. In the integrated circuit lithographic process, a photosensitive polymer film is applied to the substrate, dried, and then exposed with the proper geometrical patterns through a photomask. After exposure, the wafer is soaked in a solution that develops the images in the photosensitive material. Depending upon the type of polymer used, either exposed or non-exposed areas of film are removed in the developing process.
A frequent problem encountered in this process is reflectivity of the activating radiation back into the resist by the substrate, especially by substrates containing highly reflecting, irregular topographies. Such reflectivity tends to cause standing wave ripples and reflective notches, which interfere with the desired pattern in the photoresist. The notches are particularly bad when the support or metallurgy is nonplanar.
The problem, as well as its cause, as discovered by us, is illustrated in FIG. 1 which depicts a substrate 1 containing numerous structures appropriate to integrated circuit function whose nature and function will be described later. The substrate has been covered with an insulator 32 and patterned with a series of channels 7, which will be used for wiring interconnections in semiconductor processing. The channels 7 are to be deepened and the insulator layer 32 is to be cut back slightly to create vias, contacts or interconnections between channels. As shown in FIG. 2, a layer of photoresist has been deposited on the insulator 32 and has been exposed and developed. Because of reflected radiation during the exposure step, the development of the photoresist 36 has removed a notch 3 where a straight side was intended. As a result, when the photoresist is used as an etch mask, etching occurs in a region that was to have been protected by the resist. When the photoresist is stripped, the insulator layer 32 appears as shown in FIG. 3 having a portion of the insulator etched away where a vertical wall had been intended. When the channel is filled with a suitable metal and planarized, the metal 46 has an extension 5 which intrudes into the region which was supposed to be occupied by insulator. The extension 5 gives rise to shorting and circuit failure, particularly when the dimensions are critical.
The art discloses three basic approaches to the problem: (1) decrease the intensity or time of exposure to the radiation; (2) incorporate a radiation absorber into the photoresist; and (3) incorporate a dye or other chromophore of appropriate absorption into a removable polymer layer beneath the resist. Each of these approaches has its attendant drawbacks: Decreasing the intensity or duration of the exposure decreases reflective radiation but also decreases the radiation necessary for efficient exposure. This becomes an acute problem when creating sub-micron features. The result of underexposure is blocked or partially blocked contact/via openings due to residual resists remaining in the pattern after photodeveloping and subsequent etching. The incorporation of a radiation absorber into the photoresist itself cuts down on reflected radiation but also results in decreased resist sensitivity. In the third approach, the radiation absorbing layer beneath the resist has, in the prior art, been of such a nature that its removal was required prior to any subsequent deposition of metal. This adds additional steps to the process, and removal of the absorber can affect exposed regions in the contact/via openings.
There thus remains a continuing need for a process that precisely defines metallization in an insulator layer and that avoids the foregoing drawbacks.